I always assumed it was firecode

(i.e. a fire in the bathroom creates a vacuum which prevents the door from opening outward. maybe that’s idiotic)

I only know what the Internet says so this may or may not be correct.

1. Inward opening is probably less inclined to disperse bathroom air into the hallway outside.

2. You're probably less likely to smack someone with a door opening into a hallway. (Or course, someone entering might do it to someone exiting a bathroom but most people are probably at least somewhat trained to exercise caution under those circumstances.)

3. If anything, I'd think fire code would favor door opening out given that's the general rule.

I think doors on the emergency exit route always open outwards so that they can open in a stampede. You'd usually have push bars on them instead of a regular handle too.